Former economy minister Macron's campaign has previously complained about attempts to hack its emails, blaming Russian interests in part for the cyber attacks.

On April 26, the team said it had been the target of a attempts to steal email credentials dating back to January, but that the perpetrators had failed to compromise any campaign data.

The Kremlin has denied it was behind any such attacks, even though Macron's camp renewed complaints against Russian media and a hackers' group operating in Ukraine.

Vitali Kremez, director of research with New York-based cyber intelligence firm Flashpoint, told Reuters his review indicates that APT 28, a group tied to the GRU, the Russian military intelligence directorate, was behind the leak. He cited similarities with U.S. election hacks that have been previously attributed to that group.

APT28 last month registered decoy internet addresses to mimic the name of En Marche, which it likely used send tainted emails to hack into the campaign's computers, Kremez said. Those domains include onedrive-en-marche.fr and mail-en-marche.fr.

"If indeed driven by Moscow, this leak appears to be a significant escalation over the previous Russian operations aimed at the U.S. presidential election, expanding the approach and scope of effort from simple espionage efforts towards more direct attempts to sway the outcome," Kremez said.